1. Field of the Invention
The present invention relates to a circuit for driving a loading motor of a video cassette tape recorder, and more particularly to a low noise loading motor drive circuit for a camcorder, capable of driving the loading motor of a cassette tape at a constant velocity and thus removing uncomfortable noise.
2. Description of the Background Art
A conventional circuit for driving a loading motor in a camera-integrated video cassette tape recorder(hereinafter, referred to as a camcorder) is illustrated in FIG. 1A. As shown in the drawing, the circuit comprises a direct current power source DC of the camcorder, a constant voltage circuit 101 connected to the direct current power source DC and adapted to generate a constant voltage to be used as a voltage for driving a loading motor M, and a motor drive unit 102 adapted to supply the constant voltage from the constant voltage circuit 101 to the loading motor M, as its drive voltage. The motor drive unit 102 includes a rotation direction Control circuit for controlling the loading motor R to rotate in a normal direction CW or a reverse direction CCW, according to loading or unloading signals.
The motor drive circuit 102 is of a conventional type, comprising an integrated element. As shown in FIG. 1B, the constant voltage circuit 101 comprises a series NPN type power transistor TR101 for controlling a constant voltage and a zener diode ZD1 for supplying a reference voltage to the base of said transistor TR101 so as to predetermine a reference output of constant voltage.
The operation of the conventional loading motor drive circuit of camcorder is as follows.
Voltage from the power source DC of camcorder is converted into a constant voltage which is in turn supplied to the motor drive circuit 102. The motor drive circuit 102 supplies the constant voltage to the loading motor M as its drive voltage, according to a loading motor driving signal. At this time, the zener voltage from the zener diode ZD101 of the constant voltage circuit 101 is applied as a base voltage to the transistor TR101, so that the transistor TR101 supplies a constant voltage of a predetermined level corresponding to the amount of voltage at which the transistor TR101 turns on as determined by the zener voltage.
That is, the zener voltage of the zener diode ZD101 is set to the reference potential obtained by adding the voltage V.sub.BE between the base and the emitter of the transistor TR101 to the driving voltage of the loading motor M. By virtue of the zener voltage, a constant voltage of the level enabling a driving of loading motor M is outputted from the transistor TR101.
Thereafter, as loading or unloading of the cassette tape is controlled by the user, a loading control signal or an unloading control signal is applied to the rotation direction control circuit of the motor drive circuit 102. The rotation direction control circuit controls polarities of output voltages from the output terminals P1 and P2 of motor drive circuit unit 102, as P1 (+), P2 (-) or P1 (-), P2 (+). Accordingly, the loading motor M can rotate in the normal direction CW or the reverse direction CCW by the constant voltage from the constant voltage circuit 101.
On the other hand, FIG. 2 depicts the relationship characteristic of a load to a rotation velocity (RPM) of a small direct current motor. As shown in the drawing, the rotation velocity is inversely proportional to the load of overall mechanism.
For example, if the point B is assumed as a normal operation point, under the condition that a small direct current motor is used as loading motor M and a constant voltage is used as driving voltage, the point A is indicative of the position where the rotation velocity is increased according to the decreased load, while the point C the position where the rotation velocity is decreased according to the increased load. Thus, the rotation velocity has an inverse proportion characteristic to the load.
When controlling the loading of the overall mechanism by using a constant voltage, the load varies on the basis of the variation in the mechanical load as shown in FIG. 3A, and the loading motor M rotates at a relative rotation velocity that is inversely proportional to the load, as shown in FIG. 3B. As a result, the loading motor M does not rotate smoothly and regularly due to the load variation as shown in FIG. 3A. Such rotation irregularity results in vibration of a worm gear used as a reduction means for the loading motor and of a mechanism for supporting the worm gear, thereby irregular noise that is proportional to the variation of load occurs as show n in FIG. 3C.